Added phpstan linter for php. (#772)

* Added phpstan linter for php.

doc/ale-php.txt

@@ -43,5 +43,24 @@ g:ale_php_phpmd_ruleset                               *g:ale_php_phpmd_ruleset*
   the available phpmd rulesets
 
 
+------------------------------------------------------------------------------
+phpstan                                                          *ale-php-stan*
+
+g:ale_php_phpstan_executable                     *g:ale_php_phpstan_executable*
+                                                 *b:ale_php_phpstan_executable*
+  Type: |String|
+  Default: `'phpstan'`
+
+  This variable sets executable used for phpstan.
+
+
+g:ale_php_phpstan_level                               *g:ale_php_phpstan_level*
+                                                      *b:ale_php_phpstan_level*
+  Type: |Number|
+  Default: `4`
+
+  This variable controls the rule levels. 0 is the loosest and 4 is the
+  strictest.
+
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -74,6 +74,7 @@ CONTENTS                                                         *ale-contents*
     php...................................|ale-php-options|
       phpcs...............................|ale-php-phpcs|
       phpmd...............................|ale-php-phpmd|
+      phpstan.............................|ale-php-phpstan|
     pug...................................|ale-pug-options|
       puglint.............................|ale-pug-puglint|
     python................................|ale-python-options|
@@ -195,7 +196,7 @@ The following languages and tools are supported.
 * Objective-C++: 'clang'
 * OCaml: 'merlin' (see |ale-linter-integration-ocaml-merlin|)
 * Perl: 'perl' (-c flag), 'perlcritic'
-* PHP: 'hack', 'php' (-l flag), 'phpcs', 'phpmd'
+* PHP: 'hack', 'php' (-l flag), 'phpcs', 'phpmd', 'phpstan'
 * Pod: 'proselint'
 * Pug: 'pug-lint'
 * Puppet: 'puppet', 'puppet-lint'